The present invention relates to a sheet handling device for a printer or copier containing a print surface for supporting a first surface of a sheet, a feed plate having an edge being adjacent to the print surface, and a feed mechanism for feeding a sheet to the print surface through a gap between the edge of the feed plate and the print surface.
In printers in which paper sheets or similar image receiving sheets are used as recording media, a tendency of the paper to cockle may sometimes constitute a serious problem. The cockling phenomenon is related to the fact that paper and similar materials tend to absorb humidity from ambient air and to expand and contract in accordance with their humidity content. Typically, the expansion and contraction is not isotropic and is particularly pronounced in a direction in which the fibers of the paper are predominantly oriented. When there exists a gradient in humidity within the paper, then the more humid portion of the paper will expand more than the drier portion, which inevitably leads to the production of cockles or wrinkles.
Once cockles have developed in the paper during the transport of the paper towards the sheet support plate, a further expansion or contraction of the paper may lead to an expansion of the cockles, so that the height of the cockles also grows.
In a typical setup of an ink jet printer, especially a large format printer, the paper is intermittently advanced over a flat sheet support plate, while a carriage moves back and forth across the paper, and ink jet printheads mounted on the carriage are energized to eject droplets of ink onto the paper to form a printed image. Since the carriage moves with relatively high velocity, the ink droplets ejected onto the paper undergo a certain aberration and are deposited on the paper in a somewhat dislocated position. The amount of dislocation is proportional to the flight distance of the ink droplets. Thus, when cockles are present in the paper, the flight distance is non-uniform and, accordingly, the dislocation of the spots of ink on the paper also becomes non-uniform, so that the quality of the printed image becomes deteriorated. The larger the height of the cockles, the more pronounced is the deteriorating effect.
When the ink jet print printheads are positioned very close to the surface of the paper to minimize the dislocation, the printheads might even touch large cockles or bumps of the paper, so that the quality of the printed image is also deteriorated.